1. Field of the Invention
The present invention relates generally to an alkaline storage battery like a nickel-metal hydride battery, a hydrogen absorbing alloy electrode used as a negative electrode of the alkaline storage battery, and a hydrogen absorbing alloy used as the hydrogen absorbing alloy electrode, and is particularly characterized in that the hydrogen absorbing alloy for electrode used as the hydrogen absorbing alloy electrode and hydrogen absorbing alloy electrode are modified to improve high-rate discharge performance and charge and discharge cycle performance in the alkaline storage battery.
2. Description of the Related Art
A nickel-metal hydride battery has been conventionally known as one of alkaline storage batteries. Such a nickel-metal hydride battery has generally employed as its negative electrode a hydrogen absorbing alloy electrode.
As such a hydrogen absorbing alloy electrode for the alkaline storage battery, the one obtained by applying a paste prepared by mixing hydrogen absorbing alloy powder and a binding agent to a current collector and then drying the paste thereon has been generally utilized.
However, in the alkaline storage battery wherein the hydrogen absorbing alloy electrode as described above are used, the conductivity in the hydrogen absorbing alloy electrode is insufficient, thus charge and discharge performance at a high current is inferior. Further, the surface of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode is oxidized and deteriorated by the oxygen that appears in the positive electrode when the battery is overcharged, thus a discharge capacity is decreased gradually, and charge and discharge cycle performance is degraded.
Therefore, in Japanese Patent Laid-Open No. Shou61(1986)-163569, plating the surface of the hydrogen absorbing alloy powder with nickel or nickel alloy to improve the conductivity of hydrogen absorbing alloy powder and adhesion of the hydrogen absorbing alloy powder to the current collector has been proposed. In Japanese Patent Laid-Open No. Shou61(1986)-185863, covering the surface of the hydrogen absorbing alloy particles with carbon to prevent the oxidation of the hydrogen absorbing alloy by the oxygen that appears in the positive electrode when the battery is overcharged has been proposed. In Japanese Patent Laid-Open No. Shou63(1988)-266767, forming a porous conductive layer containing nickel and the like on the surface of the hydrogen absorbing alloy electrode to improve the conductivity of the hydrogen absorbing alloy electrode and prevent the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by the oxygen that appears in the positive electrode when the battery is overcharged has been proposed. In Japanese Patent Laid-Open No. Hei5(1993)-159798 and Japanese Patent Laid-Open No. Hei9(1997)-106817, plating the surface of the hydrogen absorbing alloy powder with metal containing particulates of water-repellent resin or fluorine compound to improve the conductivity of the hydrogen absorbing alloy powder and the adhesion of the hydrogen absorbing alloy powder to the current collector has been proposed.
Unfortunately however, even when the surface of the hydrogen absorbing alloy powder is plated with the nickel or nickel alloy, or the porous conductive layer containing the nickel and the like is formed on the surface of the hydrogen absorbing alloy electrode as mentioned above, the oxidation of the surface of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by the oxygen that appears in the positive electrode when the battery is overcharged is not prevented fully, and the charge and discharge cycle performance is not fully improved.
There remain problems that the conductivity of the hydrogen absorbing alloy electrode is decreased, and the charge and discharge performance at a high current is degraded in a case where the surface of hydrogen absorbing alloy particles is covered with carbon as mentioned above.
There still remain problems that in a part where the above-mentioned water-repellent resin or fluorine compound exists, a contact between alkaline electrolyte solution and the hydrogen absorbing alloy powder is insufficient, charge and discharge reactivity in the hydrogen absorbing alloy electrode is decreased, and the discharge performance and the charge and discharge cycle performance at a high current is degraded in the case where the surface of the hydrogen absorbing alloy particles is plated with the metal containing water-repellent resin or fluorine compound as mentioned above.
An object of the present invention is to solve the above-mentioned problems in an alkaline storage battery wherein a hydrogen absorbing alloy electrode is employed as its negative electrode.
A first object of the present invention is to modify a hydrogen absorbing alloy for electrode and the hydrogen absorbing alloy electrode, to improve conductivity of the hydrogen absorbing alloy electrode.
A second object of the present invention is to modify the hydrogen absorbing alloy for electrode and the hydrogen absorbing alloy electrode, to prevent fully the hydrogen absorbing alloy from oxidation by oxygen that appears in a positive electrode when the battery is overcharged.
A third object of the present invention is to prevent hydrogen absorbing alloy particles from falling away from a current collector in the hydrogen absorbing alloy electrode wherein an electrode material using the hydrogen absorbing alloy particles is adhered to the current collector.
A fourth object of the present invention is to improve a high-rate discharge performance and a charge and discharge cycle performance in the alkaline storage battery wherein the hydrogen absorbing alloy electrode is employed as its negative electrode.
A first hydrogen absorbing alloy for electrode according to the present invention is a hydrogen absorbing alloy for electrode containing hydrogen absorbing alloy particles, wherein a covering layer containing at least one of metal elected from nickel and cobalt, and carbon particles is formed on a surface of said hydrogen absorbing alloy particles.
The conductivity of the hydrogen absorbing alloy electrode is improved by at least one of the metal elected from the nickel and the cobalt in the covering layer formed on the surface of the hydrogen absorbing alloy particles when the hydrogen absorbing alloy electrode using the first hydrogen absorbing alloy is employed as the negative electrode of the alkaline storage battery. Further, the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode is prevented by the carbon particles in the covering layer even in the case where the oxygen appears in the positive electrode when the battery is overcharged.
The high-rate discharge performance and the charge and discharge cycle performance of the alkaline storage battery are improved as the conductivity of the hydrogen absorbing alloy electrode is improved and the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by oxygen that appears in the positive electrode when the battery is overcharged is prevented as mentioned above.
In forming the covering layer containing at least one of the metal elected from the nickel and cobalt, and the carbon particles on the surface of the hydrogen absorbing alloy particles as mentioned above, various ways can be adopted. However, to form easily the covering layer without decrease of the performance of the hydrogen absorbing alloy, the covering layer is preferably formed by plating, and more preferably by electroless plating.
In forming the covering layer containing at least one of the metal elected from the nickel and cobalt, and the carbon particles on the surface of the hydrogen absorbing alloy particles as mentioned above, when the thickness of the above-mentioned metal part in the covering layer is too thin, the conductivity of the hydrogen absorbing alloy electrode is not fully improved. On the other hand, when the thickness of the metal part is too thick, a whole surface of the hydrogen absorbing alloy particles is covered and a charge and discharge reactivity is decreased. Therefore, an average thickness of the metal part in the covering layer is preferably set in the range of 0.5 to 2.5 xcexcm.
Further, to prevent fully the oxidation by the oxygen that appears in the positive electrode when the battery is overcharged of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by the carbon particles in the covering layer, the carbon particles whose average particle diameter is not less than the average thickness of the metal part in the covering layer are preferably used, and more preferably the carbon particles whose average particle diameter is larger than the average thickness of the metal part in the covering layer.
Further, in forming the above-mentioned covering layer on the surface of the hydrogen absorbing alloy particles, when the amount of the carbon particles in the covering layer is too small, the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by the oxygen that appears in the positive electrode when the battery is overcharged is not fully prevented. On the other hand, when the amount of the carbon particles in the covering layer is too large, the conductivity of the hydrogen absorbing alloy electrode is decreased, and the charge and discharge performance at a high current is degraded. Therefore, the amount of the carbon particles in the covering layer is preferably set in the range of 0.5 to 5.0 wt %.
A first hydrogen absorbing alloy electrode according to the present invention is a hydrogen absorbing alloy electrode in which an electrode material containing hydrogen absorbing alloy particles is adhered to a current collector, wherein a covering layer containing at least one of metal elected from nickel and cobalt, and carbon particles is formed on a surface of said hydrogen absorbing alloy particles.
When the first hydrogen absorbing alloy electrode is employed as the negative electrode of the alkaline storage battery, the conductivity of the hydrogen absorbing alloy electrode is improved by at least one of the metal elected from the nickel and cobalt in the covering layer formed on the surface. Further, the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode is prevented by the carbon particles in the covering layer even in the case where the oxygen appears in the positive electrode when the battery is overcharged.
The high-rate discharge performance and the charge and discharge cycle performance of the alkaline storage battery are improved as the conductivity of the hydrogen absorbing alloy electrode is improved and the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by oxygen that appears in the positive electrode when the battery is overcharged is prevented as mentioned above.
In the first hydrogen absorbing alloy electrode, in forming the covering layer containing at least one of the metal elected from the nickel and cobalt, and the carbon particles on the surface, various ways can be adopted. However, to form easily the covering layer without the decrease of the performance of the hydrogen absorbing alloy, the covering layer is preferably formed by the plating, and more preferably by the electroless plating.
In forming the covering layer containing at least one of the metal elected from the nickel and cobalt, and the carbon particles, when the thickness of the above-mentioned metal part in the covering layer is too thin, the conductivity of the hydrogen absorbing alloy electrode is not fully improved. On the other hand, when the thickness of the metal part is too thick, the whole surface of the hydrogen absorbing alloy electrode is covered and the charge and discharge reactivity is decreased. Therefore, the average thickness of the metal part in the covering layer is preferably set in the range of 0.5 to 5.0 xcexcm.
Further, when the amount of the carbon particles in the above-mentioned covering layer is too small, the oxidation of the hydrogen absorbing alloy in the hydrogen absorbing alloy electrode by the oxygen that appears in the positive electrode when the battery is overcharged is not fully prevented. On the other hand, when the amount of the carbon particles in the covering layer is too large, the conductivity of the hydrogen absorbing alloy electrode is decreased, and the charge and discharge performance at a high current is degraded. Therefore, the amount of the carbon particles in the film layer is preferably set in the range of 0.5 to 5.0 wt %.
A second hydrogen absorbing alloy for electrode according to the present invention is a hydrogen absorbing alloy for electrode containing hydrogen absorbing alloy particles, wherein a covering layer containing at least one of metal elected from nickel and cobalt, and hydrophilic polymer is formed on a surface of said hydrogen absorbing alloy particles.
The conductivity of the hydrogen absorbing alloy electrode is improved, the contact between alkaline electrolyte solution and the hydrogen absorbing alloy particles is sufficient through the hydrophilic polymer in the covering layer, and the charge and discharge reactivity in the hydrogen absorbing alloy electrode is sufficient by at least one of the metal elected from the nickel and cobalt in the covering layer formed on the surface of the hydrogen absorbing alloy particles when the hydrogen absorbing alloy electrode using the second hydrogen absorbing alloy for electrode is employed as the negative electrode of the alkaline storage battery.
The high-rate discharge performance and the charge and discharge cycle performance of the alkaline storage battery are improved as the conductivity of the hydrogen absorbing alloy electrode is improved and the charge and discharge reactivity in the hydrogen absorbing alloy electrode is sufficient.
In the hydrogen absorbing alloy electrode wherein the second hydrogen absorbing alloy for electrode is adhered to the current collector, the adhesion of the hydrogen absorbing alloy particles to the current collector is improved and the hydrogen absorbing alloy particles are prevented from falling away from the current collector by the above-mentioned hydrophilic polymer contained in the covering layer.
In the second hydrogen absorbing alloy for electrode, in forming the covering layer containing at least one of the metal elected from the nickel and cobalt, and the hydrophilic polymer on the surface, when the thickness of the metal part of the metal elected from at least one of the nickel and cobalt in the covering layer is too thin, the conductivity of the hydrogen absorbing alloy electrode is not fully improved. On the other hand, when the thickness of the metal part is too thick, the whole surface of the hydrogen absorbing alloy particles is covered and the charge and discharge reactivity is decreased. Therefore, the average thickness of the above-mentioned metal part in the covering layer is preferably set in the range of 0.5 to 2.5 xcexcm.
Further, in forming the above-mentioned covering layer on the surface of the hydrogen absorbing alloy particles, when the amount of the hydrophilic polymer in the covering layer is too small, the adhesion of the hydrogen absorbing alloy particles to the current collector is degraded and the part where the alkaline electrolyte solution contacts with the hydrogen absorbing alloy particles through the hydrophilic polymer is decreased. On the other hand, when the amount of the hydrophilic polymer is too large, the oxidation of the hydrogen absorbing alloy particles is not sufficiently prevented. Therefore, the amount of the hydrophilic polymer in the covering layer is preferably set in the range of 0.5 to 5 wt %.
As the above-mentioned hydrophilic polymer, well-known hydrophilic polymer including hydrophilic functional group such as an ether group, a carboxy group, an aldehyde group, a ketone group, a hydroxyl group, a sulfonic acid group, an amino group, a diazo group can be employed. Examples of such hydrophilic polymer include polyethylene oxide, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylamide, polyacrylic acid, polymethacrylate, and copolymer of these, and the like.
A second hydrogen absorbing alloy electrode according to the present invention is a hydrogen absorbing alloy electrode in which an electrode material containing hydrogen absorbing alloy particles is adhered to a current collector, wherein a covering layer containing at least one of metal elected from nickel and cobalt, and hydrophilic polymer is formed on a surface of said current collector.
When the second hydrogen absorbing alloy electrode is employed as the negative electrode of the alkaline storage battery, the conductivity of the hydrogen absorbing alloy electrode is improved by at least one of the metal elected from nickel and cobalt in the covering layer formed on the surface. Further, the adhesion of the hydrogen absorbing alloy particles to the current collector is improved by the hydrophilic polymer contained in the covering layer and the hydrogen absorbing alloy particles are prevented from falling away from the current collector.
As the above-mentioned hydrophilic polymer, the hydrophilic polymer employed in the case of the above-mentioned second hydrogen absorbing alloy for electrode can be employed.
In the second hydrogen absorbing alloy electrode, when the hydrogen absorbing alloy particles wherein the covering layer containing at least one of the metal elected from the nickel and cobalt, and the hydrophilic polymer is formed on the surface is employed as the above-mentioned hydrogen absorbing alloy particles, the conductivity of the hydrogen absorbing alloy particles is further improved by at least one of the metal elected from the nickel and cobalt contained in each of the covering layers formed on the surfaces of the current collector and hydrogen absorbing alloy particles, the adhesion of the hydrogen absorbing alloy particles to the current collector is further improved and the hydrogen absorbing alloy particles are further prevented from falling away from the current collector by the hydrophilic polymer contained in each of the covering layers.
In the above-mentioned second hydrogen absorbing alloy for electrode and second hydrogen absorbing alloy electrode, in forming the covering layer containing at least one of the nickel and cobalt, and the hydrophilic polymer on the surfaces of the hydrogen absorbing alloy particles and the current collector, the covering layer can be formed by plating the surfaces of the hydrogen absorbing alloy particles and the current collector dispersing the above-mentioned hydrophilic polymer in plating fluid containing the nickel and cobalt, and in plating, it is preferable to adopt electroless plating which is simple of handling.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.